Fix GroupSetPermissionAccessEntity property load

Fix `GroupSetPermissionAccessEntity::getAccessEntityUsers()` accessing
of unset `groupset` property

- Check if `$this->groupset` is set and attempt to load it if not
  set by calling `GroupSetPermissionAccessEntity::load()`

Of note is that this could still fail with a fatal error if the gID
does not exist and the `load()` method still does not set `groupset`

http://www.concrete5.org/developers/bugs/5-6-2-1/cannot-save-versions-repost/